1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine, and a complex machine.
2. Description of the Related Art
Conventionally, there have been widely known image forming apparatuses adopting an electrophotographic method for forming an image of a document on a recording sheet. According to the electrophotographic method, a charging device uniformly charges a surface of a photoconductive drum, and an exposure device forms an electrostatic latent image corresponding to an image of a read document on the surface of the photoconductive drum. Then, toners are supplied to a portion of the electrostatic latent image so as to develop the image as a toner image, and a transfer device transfers the toner image formed on the photoconductive drum to a recording sheet.
As examples of technologies made in aim of stably maintaining a favorable state of an image forming operation performed in this kind of image forming apparatus even if a temperature change and a temporal change in a characteristic of a photoconductive member occurs, Japanese Patent Unexamined Publication No. 562-233978 (patent document 1) and Japanese Patent Unexamined Publication No. 562-235685 (patent document 2) propose technologies of changing a γ-conversion information (gradation correction information) with use of electric potential data of the photoconductive member. Further, Japanese Patent Unexamined Publication No. H9-187997 (patent document 3) discloses a technology of changing gradation correction information in accordance with at least one state detected from among states of a surface electrical potential, a developing bias, a toner density, an image density, a temperature, and a humidity.
In an image forming apparatus adopting the electrophotographic method, a developing density adjustment pattern having a predetermined print rate is read, and then a feedback control of controlling a developing bias in accordance with a density of the read developing density adjustment pattern is executed, so that a half-tone gradation is controlled. However, changes in temperature and humidity characteristics of a photoconductive drum and changes in a charging ability of a charging device due to environmental changes sometimes cause a phenomenon that the surface electrical potential of the photoconductive drum lowers. In a case where such phenomenon does not occur, a relationship between the developing bias and the toner density show a linearity. However, when the surface electrical potential of the photoconductive drum lowers as described above, the linearity in the relationship between the developing bias and the toner density is lost especially in the low gradation pattern, so that it becomes difficult to utilize the linearity to set a desirable developing bias.
Executing the correction processing with use of the γ conversion processing as disclosed in the patent documents 1, 2 causes increase in a correction processing time.